System providing ventricular pacing and biventricular coordination

ABSTRACT

A cardiac rhythm management system includes techniques for computing an indicated pacing interval, AV delay, or other timing interval. In one embodiment, a variable indicated pacing interval is computed based at least in part on an underlying intrinsic heart rate. The indicated pacing interval is used to time the delivery of biventricular coordination therapy even when ventricular heart rates are irregular, such as in the presence of atrial fibrillation. In another embodiment, a variable filter indicated AV interval is computed based at least in part on an underlying intrinsic AV interval. The indicated AV interval is used to time the delivery of atrial tracking biventricular coordination therapy when atrial heart rhythms are not arrhythmic. Other indicated timing intervals may be similarly determined. The indicated pacing interval, AV delay, or other timing interval can also be used in combination with a sensor indicated rate indicator.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application is related to the following co-pending, commonlyassigned patent applications: “Method and Apparatus for TreatingIrregular Ventricular Contractions Such as During Atrial Arrhythmia,”serial number ______, (Attorney Docket No. 00279.112US1); “CardiacRhythm Management System Promoting Atrial Pacing,” serial number ______,(Attorney Docket No. 00279.113US1); and “Cardiac Rhythm ManagementSystem With Atrial Shock Timing Optimization,” serial number ______,(Attorney Docket No. 00279.142US1); each of which are filed on even dateherewith, each of which disclosure is herein incorporated by referencein its entirety.

TECHNICAL FIELD

[0002] The present system relates generally to cardiac rhythm managementsystems and particularly, but not by way of limitation, to a systemproviding, among other things, ventricular pacing and biventricularcoordination therapy.

BACKGROUND

[0003] When functioning properly, the human heart maintains its ownintrinsic rhythm, and is capable of pumping adequate blood throughoutthe body's circulatory system. However, some people have irregularcardiac rhythms, referred to as cardiac arrhythmias. Such arrhythmiasresult in diminished blood circulation. One mode of treating cardiacarrhythmias uses drug therapy. Drugs are often effective at restoringnormal heart rhythms. However, drug therapy is not always effective fortreating arrhythmias of certain patients. For such patients, analternative mode of treatment is needed. One such alternative mode oftreatment includes the use of a cardiac rhythm management system. Suchsystems are often implanted in the patient and deliver therapy to theheart.

[0004] Cardiac rhythm management systems include, among other things,pacemakers, also referred to as pacers. Pacers deliver timed sequencesof low energy electrical stimuli, called pace pulses, to the heart, suchas via an intravascular leadwire or catheter (referred to as a “lead”)having one or more electrodes disposed in or about the heart. Heartcontractions are initiated in response to such pace pulses (this isreferred to as “capturing” the heart). By properly timing the deliveryof pace pulses, the heart can be induced to contract in proper rhythm,greatly improving its efficiency as a pump. Pacers are often used totreat patients with bradyarrhythmias, that is, hearts that beat tooslowly, or irregularly.

[0005] Cardiac rhythm management systems also include cardioverters ordefibrillators that are capable of delivering higher energy electricalstimuli to the heart. Defibrillators are often used to treat patientswith tachyarrhythmias, that is, hearts that beat too quickly. Suchtoo-fast heart rhythms also cause diminished blood circulation becausethe heart isn't allowed sufficient time to fill with blood beforecontracting to expel the blood. Such pumping by the heart isinefficient. A defibrillator is capable of delivering an high energyelectrical stimulus that is sometimes referred to as a defibrillationcountershock. The countershock interrupts the tachyarrhythmia, allowingthe heart to reestablish a normal rhythm for the efficient pumping ofblood. In addition to pacers, cardiac rhythm management systems alsoinclude, among other things, pacer/defibrillators that combine thefunctions of pacers and defibrillators, drug delivery devices, and anyother implantable or external systems or devices for diagnosing ortreating cardiac arrhythmias.

[0006] One problem faced by cardiac rhythm management systems is thetreatment of congestive heart failure (also referred to as “CHF”).Congestive heart failure, which can result from long-term hypertension,is a condition in which the muscle in the walls of at least one of theright and left sides of the heart deteriorates. By way of example,suppose the muscle in the walls of left side of the heart deteriorates.As a result, the left atrium and left ventricle become enlarged, and theheart muscle displays less contractility. This decreases cardiac outputof blood through the circulatory system which, in turn, may result in anincreased heart rate and less resting time between heartbeats. The heartconsumes more energy and oxygen, and its condition typically worsensover a period of time.

[0007] In the above example, as the left side of the heart becomesenlarged, the intrinsic electrical heart signals that control heartrhythm are also affected. Normally, such intrinsic signals originate inthe sinoatrial (SA) node in the upper right atrium, traveling throughand depolarizing the atrial heart tissue such that resultingcontractions of the right and left atria are triggered. The intrinsicatrial heart signals are received by the atrioventricular (AV) nodewhich, in turn, triggers a subsequent ventricular intrinsic heart signalthat travels through and depolarizes the ventricular heart tissue suchthat resulting contractions of the right and left ventricles aretriggered substantially simultaneously.

[0008] In the above example, where the left side of the heart has becomeenlarged due to congestive heart failure, however, the ventricularintrinsic heart signals may travel through and depolarize the left sideof the heart more slowly than in the right side of the heart. As aresult, the left and right ventricles do not contract simultaneously,but rather, the left ventricle contracts after the right ventricle. Thisreduces the pumping efficiency of the heart. Moreover, in the case ofleft bundle branch block (LBBB), for example, different regions withinthe left ventricle may not contract together in a coordinated fashion.

[0009] Congestive heart failure can be treated by biventricularcoordination therapy that provides pacing pulses to both right and leftventricles. See, e.g. Mower U.S. Pat. No. 4,928,688. Congestive heartfailure may also result in an overly long atrioventricular (AV) delaybetween atrial and ventricular contractions, again reducing the pumpingefficiency of the heart. There is a need to provide congestive heartfailure patients with improved pacing and coordination therapies forimproving the AV delay, coordinating ventricular contractions, orotherwise increasing heart pumping efficiency.

[0010] Another problem faced by cardiac rhythm management systems is thepresence of atrial tachyarrhythmias, such as atrial fibrillation,occurring in patients having congestive heart failure. Atrialfibrillation is a common cardiac arrhythmia that reduces the pumpingefficiency of the heart, though not to as great a degree as inventricular fibrillation. However, this reduced pumping efficiencyrequires the ventricle to work harder, which is particularly undesirablein congestive heart failure or other sick patients that cannot tolerateadditional stresses. Even though a congestive heart failure may haveadequate ventricular coordination and cardiac output in the presence ofa normal sinus rhythm, when atrial tachyarrhythmia is present,ventricular incoordination may occur, seriously worsening cardiacfunction.

[0011] Moreover, some devices treating congestive heart failure senseatrial heart rate and provide biventricular coordination therapy at aventricular heart rate that tracks the atrial heart rate. See, e.g.,Mower U.S. Pat. No. 4,928,688. Such atrial-tracking devices require anormal sinus rhythm to ensure proper delivery of biventricularcoordination therapy. In the presence of atrial tachyarrhythmias, suchas atrial fibrillation, however, such atrial tracking biventricularcoordination therapy could lead to too-fast and irregular biventricularcoordination that is ineffective and even dangerous.

[0012] Another problem is that atrial fibrillation may induce irregularventricular heart rhythms by processes that are yet to be fullyunderstood. Such induced ventricular arrhythmias compromise pumpingefficiency even more drastically than atrial arrhythmias. Some devicestreating congestive heart failure provide biventricular coordinationtherapy that does not track the atrial heart rate, but instead, a sensedventricular contraction in a first ventricle triggers a ventricular pacein the other ventricle, or in both ventricles. See, e.g., Mower U.S.Pat. No. 4,928,688. Even if such biventricular coordination therapy isventricular-triggered rather than atrial-tracking, the presence ofatrial tachyarrhythmias could lead to ventricular arrhythmias, such thatthe biventricular coordination therapy becomes ineffective and evendangerous because it is too-fast or irregular because of the irregularventricular heart rate. For these and other reasons, there is a need toprovide congestive heart failure patients with improved pacing andcoordination therapies for improving the AV delay, coordinatingventricular contractions, or otherwise increasing heart pumpingefficiency, even during atrial arrhythmias such as atrial fibrillation.

SUMMARY OF THE INVENTION

[0013] The present system provides, among other things, a cardiac rhythmmanagement system including techniques for computing an indicated pacinginterval, AV delay, or other timing interval. In one embodiment, avariable indicated pacing interval is computed based at least in part onan underlying intrinsic heart rate. The indicated pacing interval isused to time the delivery of biventricular coordination therapy evenwhen ventricular heart rates are irregular, such as in the presence ofatrial fibrillation. In another embodiment, a variable filter indicatedAV interval is computed based at least in part on an underlyingintrinsic AV interval. The indicated AV interval is used to time thedelivery of atrial tracking biventricular coordination therapy whenatrial heart rhythms are not arrhythmic. Other indicated timingintervals may be similarly determined. The indicated pacing interval, AVdelay, or other timing interval can also be used in combination with asensor indicated rate indicator.

[0014] In one embodiment, the system includes a first method. Actualtiming intervals between cardiac events are obtained. The systemcomputes a first indicated timing interval based at least on a mostrecent actual timing interval duration and a previous value of the firstindicated timing interval. The system provides pacing therapy based onthe first indicated timing interval.

[0015] In another embodiment, the system includes a second method. Thesystem obtains atrio-ventricular (AV) intervals between atrial eventsand successive ventricular events. The system computes a first indicatedAV interval based at least on a most recent AV interval duration and aprevious value of the first indicated AV interval. The system providespacing therapy based on the first indicated AV interval.

[0016] In another embodiment, the system includes a third method. Thesystem obtains V-V intervals between ventricular beats. The systemcomputes a first indicated pacing interval based at least on a mostrecent V-V interval duration and a previous value of the first indicatedpacing interval. The system provides pacing therapy to first and secondventricles, based on the first indicated pacing interval.

[0017] Another embodiment provides a cardiac rhythm management systemthat includes a sensing circuit for sensing events, a controllerobtaining timing intervals between events and computing a firstindicated timing interval based at least on a most recent actual timinginterval duration and a previous value of the first indicated timinginterval, and a therapy circuit, providing pacing therapy based on thefirst indicated timing interval.

[0018] Another embodiment provides a cardiac rhythm management systemthat includes at least one ventricular sensing circuit, a controller,and a ventricular therapy circuit. The controller includes a V-Vinterval timer obtaining V-V intervals between successive events in atleast one ventricle, a first register for storing a first indicatedpacing interval, and a filter updating the first indicated pacinginterval based on a most recent V-V interval provided by the VV intervaltimer and previous value of the first indicated pacing interval storedthe first register. The ventricular therapy circuit provides pacingtherapy to first and second ventricles based at least partially on thefirst indicated pacing interval. Other aspects of the invention will beapparent on reading the following detailed description of the inventionand viewing the drawings that form a part thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

[0019] In the drawings, like numerals describe substantially similarcomponents throughout the several views. Like numerals having differentletter suffixes represent different instances of substantially similarcomponents.

[0020]FIG. 1 is a schematic drawing illustrating generally oneembodiment of portions of a cardiac rhythm management system and anenvironment in which it is used.

[0021]FIG. 2 is a schematic drawing illustrating one embodiment of acardiac rhythm management device coupled by leads to a heart.

[0022]FIG. 3 is a schematic diagram illustrating generally oneembodiment of portions of a cardiac rhythm management device coupled toa heart.

[0023]FIG. 4 is a schematic diagram illustrating generally oneembodiment of a controller.

[0024]FIG. 5 is a schematic diagram illustrating generally oneconceptualization of portions of a controller.

[0025]FIG. 6 is a signal flow diagram illustrating generally oneconceptual embodiment of operating a filter.

[0026]FIG. 7 is a signal flow diagram illustrating generally anotherconceptualization of operating the filter.

[0027]FIG. 8 is a signal flow diagram illustrating generally a furtherconceptualization of operating the filter.

[0028]FIG. 9 is a schematic diagram illustrating generally anotherconceptualization of portions of a controller.

[0029]FIG. 10 is a schematic diagram illustrating generally a furtherconceptualization of portions of a controller.

[0030]FIG. 11 is a graph illustrating generally one embodiment ofoperating a filter to provide a first indicated pacing rate, such as aVRR indicated rate, for successive ventricular heart beats.

[0031]FIG. 12 is a graph illustrating generally another embodiment ofoperating a filter to provide the first indicated pacing rate, such as aVRR indicated rate, and delivering therapy based on the first indicatedpacing rate and based on a second indicated pacing rate, such as asensor indicated rate.

[0032]FIG. 13 is a graph illustrating generally another illustrativeexample of heart rate vs. time according to a VRR algorithm spreadsheetsimulation.

[0033]FIG. 14 is a graph illustrating generally one embodiment of usingat least one of coefficients a and b as a function of heart rate (or acorresponding time interval).

[0034]FIG. 15 is a schematic drawing, similar to FIG. 2, illustratinggenerally one embodiment of a cardiac rhythm management device coupledby leads to a heart, such as for providing biventricular coordinationtherapy.

[0035]FIG. 16 is a schematic drawing, similar to FIG. 3, illustratinggenerally one embodiment of portions of a cardiac rhythm managementdevice including, among other things, left ventricular sensing andtherapy circuits.

[0036]FIG. 17 is a schematic drawing, similar to FIG. 5, illustratinggenerally portions of one conceptual embodiment of a controller.

[0037]FIG. 18 is a schematic diagram, similar to FIG. 17, illustratinggenerally another conceptualization of portions of a controller used forregulating an AV interval based on a filter indicated AV delay.

DETAILED DESCRIPTION

[0038] In the following detailed description, reference is made to theaccompanying drawings which form a part hereof, and in which is shown byway of illustration specific embodiments in which the invention may bepracticed. These embodiments are described in sufficient detail toenable those skilled in the art to practice the invention, and it is tobe understood that the embodiments may be combined, or that otherembodiments may be utilized and that structural, logical and electricalchanges may be made without departing from the spirit and scope of thepresent invention. The following detailed description is, therefore, notto be taken in a limiting sense, and the scope of the present inventionis defined by the appended claims and their equivalents. In thedrawings, like numerals describe substantially similar componentsthroughout the several views. Like numerals having different lettersuffixes represent different instances of substantially similarcomponents.

[0039] The present methods and apparatus will be described inapplications involving implantable medical devices including, but notlimited to, implantable cardiac rhythm management systems such aspacemakers, cardioverter/defibrillators, pacer/defibrillators, andbiventricular or other multi-site coordination devices. However, it isunderstood that the present methods and apparatus may be employed inunimplanted devices, including, but not limited to, external pacemakers,cardioverter/defibrillators, pacer/defibrillators, biventricular orother multi-site coordination devices, monitors, programmers andrecorders.

Problems Associated With Atrial Arrhythmias

[0040] As stated earlier, one potential cause of irregularity ofventricular contractions arises during atrial tachyarrhythmias, such asatrial fibrillation. During atrial fibrillation, irregular ventricularcontractions may be caused by an atrial tachyarrhythmia that isconducted to the ventricles. Pacing the ventricles regularizes theventricular heart rate by establishing retrograde conduction from theventricles. This, in turn, is believed to block forward conduction ofatrial signals through the atrioventricular (A-V) node. As a result,irregular atrial signals do not trigger resulting irregular ventricularcontractions.

[0041] One therapy for treating irregular ventricular contractionsduring atrial fibrillation is to increase the ventricular heart rate bypacing the ventricles at a higher rate than the average unpaced(intrinsic) ventricular heart rate. Such therapy improves cardiac outputbecause it stabilizes the rate of ventricular contractions to avoidshort periods between contractions and/or long periods without acontraction. Such therapy is also believed to decrease the ability ofthe atrial fibrillation to induce irregular ventricular contractions.Additionally, pacing the ventricles at above the average intrinsicventricular heart rate can provide coordination therapy. Coordinationtherapy applies the pacing stimulation to one ventricle at multiplesites, to both ventricles at a single site in each ventricle, or to bothventricles at multiple sites in each ventricle. Coordination therapy isapplied to the sites in a fashion that coordinates the sequence ofcontraction in ventricular heart tissue. Coordination therapy isbelieved to increase systolic pulse pressure in patients withventricular conduction disorders, such as left bundle branch block(LBBB), associated with uncoordinated ventricular contractions.Coordination therapy also decreases the time required for systoliccontraction, leaving more time for diastolic ventricular filling,thereby also improving the end diastolic pressure.

Ventricular Rate Regularization (VRR) Example

[0042] This document describes, among other things, a cardiac rhythmmanagement system providing a method and apparatus for treatingirregular ventricular contractions during atrial arrhythmia by activelystabilizing the ventricular heart rate to obtain less potentiallyproarrhythmic conditions for delivering the atrial tachyarrhythmiatherapy. One suitable technique for stabilizing ventricular heart rateis referred to as Ventricular Rate Regularization, described in Krig etal. U.S. patent application Ser. No. ______ entitled “Method andApparatus for Treating Irregular Ventricular Contractions Such As DuringAtrial Arrhythmia,” which is filed on even date herewith, assigned tothe assignee of the present patent application, and which is hereinincorporated by reference in its entirety.

[0043]FIG. 1 is a schematic drawing illustrating, by way of example, butnot by way of limitation, one embodiment of portions of a cardiac rhythmmanagement system 100 and an environment in which it is used. In FIG. 1,system 100 includes an implantable cardiac rhythm management device 105,also referred to as an electronics unit, which is coupled by anintravascular endocardial lead 110, or other lead, to a heart 115 ofpatient 120. System 100 also includes an external programmer 125providing wireless communication with device 105 using a telemetrydevice 130. Catheter lead 110 includes a proximal end 135, which iscoupled to device 105, and a distal end 140, which is coupled to one ormore portions of heart 115.

[0044]FIG. 2 is a schematic drawing illustrating, by way of example, butnot by way of limitation, one embodiment of device 105 coupled by leads110A-B to heart 115, which includes a right atrium 200A, a left atrium200B, a right ventricle 205A, a left ventricle 205B, and a coronarysinus 220 extending from right atrium 200A. In this embodiment, atriallead 110A includes electrodes (electrical contacts) disposed in, around,or near an atrium 200 of heart 115, such as ring electrode 225 and tipelectrode 230, for sensing signals and/or delivering pacing therapy tothe atrium 200. Lead 110A optionally also includes additionalelectrodes, such as for delivering atrial and/or ventricularcardioversion/defibrillation and/or pacing therapy to heart 115.

[0045] In FIG. 2, a ventricular lead 110B includes one or moreelectrodes, such as tip electrode 235 and ring electrode 240, fordelivering sensing signals and/or delivering pacing therapy. Lead 110Boptionally also includes additional electrodes, such as for deliveringatrial and/or ventricular cardioversion/defibrillation and/or pacingtherapy to heart 115. Device 105 includes components that are enclosedin a hermetically-sealed can 250. Additional electrodes may be locatedon the can 250, or on an insulating header 255, or on other portions ofdevice 105, for providing unipolar pacing and/or defibrillation energyin conjunction with the electrodes disposed on or around heart 115.Other forms of electrodes include meshes and patches which may beapplied to portions of heart 115 or which may be implanted in otherareas of the body to help “steer” electrical currents produced by device105. The present method and apparatus will work in a variety ofconfigurations and with a variety of electrical contacts or“electrodes.”

Example Cardiac Rhythm Management Device

[0046]FIG. 3 is a schematic diagram illustrating generally, by way ofexample, but not by way of limitation, one embodiment of portions ofdevice 105, which is coupled to heart 115. Device 105 includes a powersource 300, an atrial sensing circuit 305, a ventricular sensing circuit310, a ventricular therapy circuit 320, and a controller 325.

[0047] Atrial sensing circuit 305 is coupled by atrial lead 110A toheart 115 for receiving, sensing, and/or detecting electrical atrialheart signals. Such atrial heart signals include atrial activations(also referred to as atrial depolarizations or P-waves), whichcorrespond to atrial contractions. Such atrial heart signals includenormal atrial rhythms, and abnormal atrial rhythms including atrialtachyarrhythmias, such as atrial fibrillation, and other atrialactivity. Atrial sensing circuit 305 provides one or more signals tocontroller 325, via node/bus 327, based on the received atrial heartsignals. Such signals provided to controller 325 indicate, among otherthings, the presence of atrial fibrillation.

[0048] Ventricular sensing circuit 310 is coupled by ventricular lead110B to heart 115 for receiving, sensing, and/or detecting electricalventricular heart signals, such as ventricular activations (alsoreferred to as ventricular depolarizations or R-waves), which correspondto ventricular contractions. Such ventricular heart signals includenormal ventricular rhythms, and abnormal ventricular rhythms, includingventricular tachyarrhythmias, such as ventricular fibrillation, andother ventricular activity, such as irregular ventricular contractionsresulting from conducted signals from atrial fibrillation. Ventricularsensing circuit 310 provides one or more signals to controller 325, vianode/bus 327, based on the received ventricular heart signals. Suchsignals provided to controller 325 indicate, among other things, thepresence of ventricular depolarizations, whether regular or irregular inrhythm.

[0049] Ventricular therapy circuit 320 provides ventricular pacingtherapy, as appropriate, to electrodes located at or near one of theventricles 205 of heart 115 for obtaining resulting evoked ventriculardepolarizations. In one embodiment, ventricular therapy circuit 320 alsoprovides cardioversion/defibrillation therapy, as appropriate, toelectrodes located at or near one of the ventricles 205 of heart 115,for terminating ventricular fibrillation and/or other ventriculartachyarrhythmias.

[0050] Controller 325 controls the delivery of therapy by ventriculartherapy circuit 320 and/or other circuits, based on heart activitysignals received from atrial sensing circuit 305 and ventricular sensingcircuit 310, as discussed below. Controller 325 includes variousmodules, which are implemented either in hardware or as one or moresequences of steps carried out on a microprocessor or other controller.Such modules are illustrated separately for conceptual clarity; it isunderstood that the various modules of controller 325 need not beseparately embodied, but may be combined and/or otherwise implemented,such as in software/firmware.

[0051] In general terms, sensing circuits 305 and 310 sense electricalsignals from heart tissue in contact with the catheter leads 110A-B towhich these sensing circuits 305 and 310 are coupled. Sensing circuits305 and 310 and/or controller 325 process these sensed signals. Based onthese sensed signals, controller 325 issues control signals to therapycircuits, such as ventricular therapy circuit 320, if necessary, for thedelivery of electrical energy (e.g., pacing and/or defibrillationpulses) to the appropriate electrodes of leads 110A-B. Controller 325may include a microprocessor or other controller for execution ofsoftware and/or firmware instructions. The software of controller 325may be modified (e.g., by remote external programmer 105) to providedifferent parameters, modes, and/or functions for the implantable device105 or to adapt or improve performance of device 105.

[0052] In one further embodiment, one or more sensors, such as sensor330, may serve as inputs to controller 325 for adjusting the rate atwhich pacing or other therapy is delivered to heart 115. One such sensor330 includes an accelerometer that provides an input to controller 325indicating increases and decreases in physical activity, for whichcontroller 325 increases and decreases pacing rate, respectively.Another such sensor includes an impedance measurement, obtained frombody electrodes, which provides an indication of increases and decreasesin the patient's respiration, for example, for which controller 325increases and decreases pacing rate, respectively. Any other sensor 330providing an indicated pacing rate can be used.

[0053]FIG. 4 is a schematic diagram illustrating generally, by way ofexample, but not by way of limitation, one embodiment of controller 325that includes several different inputs to modify the rate at whichpacing or other therapy is delivered. For example, Input #1 may provideinformation about left ventricular rate, Input #2 may provide anaccelerometer-based indication of activity, and Input #3 may provide animpedance-based indication of respiration, such as minute ventilation.Based on at least one of these and/or other inputs, controller 325provides an output indication of pacing rate as a control signaldelivered to a therapy circuit, such as to ventricular therapy circuit320. Ventricular therapy circuit 320 issues pacing pulses based on oneor more such control signals received from controller 325. Control ofthe pacing rate may be performed by controller 325, either alone or incombination with peripheral circuits or modules, using software,hardware, firmware, or any combination of the like. The softwareembodiments provide flexibility in how inputs are processed and may alsoprovide the opportunity to remotely upgrade the device software whilestill implanted in the patient without having to perform surgery toremove and/or replace the device 105.

Controller Example 1

[0054]FIG. 5 is a schematic diagram illustrating generally, by way ofexample, but not by way of limitation, one conceptualization of portionsof controller 325. At least one signal from ventricular sensing circuit310 is received by ventricular event module 500, which recognizes theoccurrence of ventricular events included within the signal. Such eventsare also referred to as “beats,” “activations,” “depolarizations,” “QRScomplexes,” “R-waves,” “contractions. ” Ventricular event module 500detects intrinsic events (also referred to as sensed events) from thesignal obtained from ventricular sensing circuit 310. Ventricular eventmodule 500 also detects evoked events (resulting from a pace) eitherfrom the signal obtained from ventricular sensing circuit 310, orpreferably from a ventricular pacing control signal obtained from pacingcontrol module 505, which also triggers the delivery of a pacingstimulus by ventricular therapy circuit 320. Thus, ventricular eventsinclude both intrinsic/sensed events and evoked/paced events.

[0055] A time interval between successive ventricular events, referredto as a V-V interval, is recorded by a first timer, such as V-V intervaltimer 510. A filter 515 computes a “first indicated pacing interval,”i.e., one indication of a desired time interval between ventricularevents or, stated differently, a desired ventricular heart rate. Thefirst indicated pacing interval is also referred to as a ventricularrate regularization (VRR) indicated pacing interval. In variousembodiments, filter 515 includes an averager, a weighted averager, amedian filter, an infinite (IIR) filter, a finite impulse response (FIR)filter, or any other analog or digital signal processing circuitproviding the desired signal processing described more particularlybelow.

[0056] In one embodiment, filter 515 computes a new value of the firstindicated pacing interval based on the duration of the most recent V-Vinterval recorded by timer 510 and on a previous value of the firstindicated pacing interval stored in first indicated pacing intervalregister 520. Register 520 is then updated by storing the newly computedfirst indicated pacing interval in register 520. Based on the firstindicated pacing interval stored in register 520, pacing control module505 delivers control signals to ventricular therapy circuit 320 fordelivering therapy, such as pacing stimuli, at the VRR-indicatedventricular heart rate corresponding to the inverse of the duration ofthe first indicated pacing interval.

Filter Example 1

[0057] In general terms, for one embodiment, device 105 obtains V-Vintervals between successive sensed or evoked ventricular beats. Device105 computes a new first indicated pacing interval based at least inpart on the duration of the most recent V-V interval and a previousvalue of the first indicated pacing interval. Device 105 provides pacingtherapy delivered at a rate corresponding to the inverse of the durationof the first indicated pacing interval.

[0058]FIG. 6 is a signal flow diagram illustrating generally, by way ofexample, but not by way of limitation, one embodiment of operatingfilter 515. Upon the occurrence of a sensed or evoked ventricular beat,timer 510 provides filter 515 with the duration of the V-V intervalconcluded by that beat, which is referred to as the most recent V-Vinterval (VV_(n)). Filter 515 also receives the previous value of thefirst indicated pacing interval (T_(n-1)) stored in register 520. Themost recent V-V interval VV_(n) and the previous value of the firstindicated pacing interval T_(n-1) are each scaled by respectiveconstants A and B, and then summed to obtain a new value of the firstindicated pacing interval (T_(n)), which is stored in register 520 andprovided to pacing control module 505. In one embodiment, thecoefficients A and B are different values, and are either programmable,variable, or constant.

[0059] If no ventricular beat is sensed during the new first indicatedpacing interval T_(n), which is measured as the time from the occurrenceof the ventricular beat concluding the most recent V-V interval VV_(n),then pacing control module 505 instructs ventricular therapy circuit 320to deliver a ventricular pacing pulse upon the expiration of the newfirst indicated pacing interval T_(n). In one embodiment, operation ofthe filter is described by T_(n)=A·VV_(n)+B·T_(n-1), where A and B arecoefficients (also referred to as “weights”), VV_(n) is the most recentV-V interval duration, and T_(n-1) is the previous value of the firstindicated pacing interval.

[0060] Initialization of filter 515 includes seeding the filter bystoring, in register 520, an initial interval value. In one embodiment,register 520 is initialized to an interval value corresponding to alower rate limit (LRL), i.e., a minimum rate at which pacing pulses aredelivered by device 105. Register 520 could alternatively be initializedwith any other suitable value.

Filter Example 2

[0061] In one embodiment, operation of filter 515 is based on whetherthe beat concluding the most recent V-V interval VV_(n) is asensed/intrinsic beat or a paced/evoked beat. In this embodiment, thepacing control module 505, which controls the timing and delivery ofpacing pulses, provides an input to filter 515 that indicates whetherthe most recent V-V interval VV_(n) was concluded by an evoked beatinitiated by a pacing stimulus delivered by device 105, or was concludedby an intrinsic beat sensed by ventricular sensing circuit 310.

[0062] In general terms, if the most recent V-V interval VV_(n) isconcluded by a sensed/intrinsic beat, then filter 515 provides a newfirst indicated pacing interval T_(n) that is adjusted from the value ofthe previous first indicated pacing interval T_(n-1) such as, forexample, decreased by an amount that is based at least partially on theduration of the most recent V-V interval VV_(n) and on the duration ofthe previous value of the first indicated pacing interval T_(n-1). If,however, the most recent V-V interval VV_(n) is concluded by apaced/evoked beat, then filter 515 provides a new first indicated pacinginterval T_(n) that is increased from the value of the previous firstindicated pacing interval T_(n-1), such as, for example, by an amountthat is based at least partially on the duration of the most recent V-Vinterval VV_(n) and on the duration of the previous value of the firstindicated pacing interval T_(n-1). If no ventricular beat is sensedduring the new first indicated pacing interval T_(n), which is measuredas the time from the occurrence of the ventricular beat concluding themost recent V-V interval VV_(n), then pacing control module 505instructs ventricular therapy circuit 320 to deliver a ventricularpacing pulse upon the expiration of the new first indicated pacinginterval T_(n).

[0063]FIG. 7 is a signal flow diagram, illustrating generally, by way ofexample, but not by way of limitation, another conceptualization ofoperating filter 515, with certain differences from FIG. 6 moreparticularly described below. In this embodiment, the pacing controlmodule 505, which controls the timing and delivery of pacing pulses,provides an input to filter 515 that indicates whether the most recentV-V interval VV_(n) was concluded by an evoked beat initiated by apacing stimulus delivered by device 105, or was concluded by anintrinsic beat sensed by ventricular sensing circuit 310.

[0064] If the most recent V-V interval VV_(n) was concluded by anintrinsic beat, then the most recent V-V interval VV_(n) and theprevious value of the first indicated pacing interval T_(n-1) are eachscaled by respective constants A and B, and then summed to obtain thenew value of the first indicated pacing interval T_(n), which is storedin register 520 and provided to pacing control module 505.Alternatively, if the most recent V-V interval VV_(n) was concluded by aevoked/paced beat, then the most recent V-V interval VV_(n) and theprevious value of the first indicated pacing interval T_(n-1) are eachscaled by respective constants C and D, and then summed to obtain thenew value of the first indicated pacing interval T_(n), which is storedin register 520 and provided to pacing control module 505. In oneembodiment, the coefficients C and D are different from each other, andare either programmable, variable, or constant. In a further embodiment,the coefficient C is a different value from the coefficient A, and/orthe coefficient D is a different value than the coefficient B, and thesecoefficients are either programmable, variable, or constant. In anotherembodiment, the coefficient D is the same value as the coefficient B.

[0065] In one embodiment, operation of filter 515 is described byT_(n)=A·VV_(n)+B·T_(n-1), if VV_(n) is concluded by an intrinsic beat,and is described by T_(n)=C·VV_(n)+D·T_(n), if VV_(n) is concluded by apaced beat, where A, B, C and D are coefficients (also referred to as“weights”), VV_(n) is the most recent V-V interval duration, T_(n) isthe new value of the first indicated pacing interval, and T_(n-1) is theprevious value of the first indicated pacing interval. If no ventricularbeat is sensed during the new first indicated pacing interval T_(n),which is measured as the time from the occurrence of the ventricularbeat concluding the most recent V-V interval VV_(n), then pacing controlmodule 505 instructs ventricular therapy circuit 320 to deliver aventricular pacing pulse upon the expiration of the new first indicatedpacing interval T_(n).

Filter Example 3

[0066] In another embodiment, these coefficients can be moreparticularly described using an intrinsic coefficient (a), a pacedcoefficient (b), and a weighting coefficient (w). In one suchembodiment, A=a·w, B=(1−w), C=b·w, and D=(1−w). In one example,operation of the filter 515 is described byT_(n)=a·w·VV_(n)+(1−w)·T_(n), if VV_(n) is concluded by an intrinsicbeat, otherwise is described by T_(n)=b·w·VV_(n)+(1−w)·T_(n-1), ifVV_(n) is concluded by a paced beat, as illustrated generally, by way ofexample, but not by way of limitation, in the signal flow graph of FIG.8. If no ventricular beat is sensed during the new first indicatedpacing interval T_(n), which is measured as the time from the occurrenceof the ventricular beat concluding the most recent V-V interval VV_(n),then pacing control module 505 instructs ventricular therapy circuit 320to deliver a ventricular pacing pulse upon the expiration of the newfirst indicated pacing interval T_(n). In one embodiment, thecoefficients a and b are different from each other, and are eitherprogrammable, variable, or constant.

[0067] The above-described parameters (e.g., A, B, C, D, a, b, w) arestated in terms of time intervals (e.g., VV_(n), T_(n),T_(n-1)).However, an alternate system may produce results in terms ofrate, rather than time intervals, without departing from the presentmethod and apparatus. In one embodiment, weighting coefficient w,intrinsic coefficient a, and paced coefficient b, are variables.Different selections of w, a, and b, will result in different operationof the present method and apparatus. For example, as w increases theweighting effect of the most recent V-V interval VV_(n) increases andthe weighting effect of the previous first indicated pacing rate T_(n-1)decreases. In one embodiment, w={fraction (1/16)}=0.0625. In anotherembodiment, w={fraction (1/32)}. Another possible range for w is fromw=½ to w={fraction (1/1024)}. A further possible range for w is from w≈0to w≈1. Other values of w, which need not include division by powers oftwo, may be substituted without departing from the present method andapparatus.

[0068] In one embodiment, intrinsic coefficient a, is selected to begreater than 0.5, or to be greater than 1.0. In one example, theintrinsic coefficient a is selected to be lesser in value than thepacing coefficient b. In one example, a≈1.1 and b≈1.2. In anotherembodiment a=0.9 and b=1.1. One possible range for a is from a=0.5 toa=2.0, and for b is from b=1.0 to b=3.0. The coefficients may varywithout departing from the present method and apparatus.

[0069] In one embodiment, for b>1 and for substantially regular V-Vintervals, filter 515 provides a new first indicated pacing intervalT_(n) that is at least slightly longer than the expected intrinsic V-Vinterval being measured by timer 515. Thus, if the intrinsic V-Vinterval being timed is consistent with the duration of previouslyreceived V-V intervals, then filter 515 avoids triggering a pacingstimulus. In such a case, a pacing pulse is delivered only if thepresently timed V-V interval becomes longer than the previoussubstantially constant V-V intervals. In general terms, filter 515operates so that pacing pulses are typically inhibited if theventricular rate is substantially constant. However, if the measured V-Vintervals become irregular, then filter 515 operates, over a period ofone or several such V-V intervals, to shorten the first indicated pacinginterval T_(n) so that pacing stimuli are being delivered.

[0070] According to one aspect of the invention, it is believed that ifthe irregular V-V intervals are caused by a conducted atrialtachyarrhythmia, then pacing the ventricle will regularize theventricular heart rate by establishing retrograde conduction from theventricle. This, in turn, blocks forward conduction of atrial signalsthrough the atrioventricular (A-V) node. As a result, irregular atrialsignals do not trigger resulting irregular ventricular contractions.According to another aspect of the invention, however, this method andapparatus will not introduce pacing pulses until the heartbeat becomesirregular. Therefore, the heart is assured to pace at its intrinsic ratewhen regular ventricular contractions are sensed.

Controller Example 2

[0071]FIG. 9 is a schematic diagram illustrating generally, by way ofexample, but not by way of limitation, another conceptualization ofportions of controller 325, with certain differences from FIG. 5 moreparticularly described below. In FIG. 9, controller 325 receives fromsensor 330 a signal including information from which a physiologicallydesired heart rate (e.g., based on the patient's activity, respiration,or any other suitable indicator of metabolic need) can be derived. Thesensor signal is digitized by an A/D converter 900. The digitized signalis processed by a sensor rate module 905, which computes a desired heartrate that is expressed in terms of a second indicated pacing intervalstored in register 910.

[0072] Pacing control module 505 delivers a control signal, whichdirects ventricular therapy circuit 320 to deliver a pacing pulse, basedon either (or both) of the first or second indicated pacing intervals,stored in registers 520 and 910, respectively, or both. In oneembodiment, pacing control module 505 includes a selection module 915that selects between the new first indicated pacing interval T_(n) andthe sensor-based second indicated pacing interval.

[0073] In one embodiment, selection module 915 selects the shorter ofthe first and second indicated pacing intervals as the selectedindicated pacing interval S_(n). If no ventricular beat is sensed duringthe selected indicated pacing interval S_(n), which is measured as thetime from the occurrence of the ventricular beat concluding the mostrecent V-V interval VV_(n), then pacing control module 505 instructsventricular therapy circuit 320 to deliver a ventricular pacing pulseupon the expiration of the selected indicated pacing interval S_(n).

[0074] In general terms, for this embodiment, the ventricle is paced atthe higher of the sensor indicated rate and the VRR indicated rate. If,for example, the patient is resting, such that the sensor indicated rateis lower than the patient's intrinsic rate, and the patient's intrinsicrate is substantially constant, then the intrinsic rate is higher thanthe VRR indicated rate. As a result, pacing pulses generally will not bedelivered. But if, for example, the patient is resting, but with anatrial tachyarrhythmia that induces irregular ventricular contractions,then pacing pulses generally will be delivered at the VRR indicatedrate. In another example, if the patient is active, such that the sensorindicated rate is higher than the VRR indicated rate, then pacing pulsesgenerally will be delivered at the sensor indicated rate. In analternative embodiment, the pacing rate is determined by blending thesensor indicated rate and the VRR indicated rate, rather than byselecting the higher of these two indicated rates (i.e., the shorter ofthe first and second indicated pacing intervals).

[0075] In another embodiment, selection module 915 provides a selectedindicated pacing interval S_(n) based on a blending of both the firstand second indicated pacing intervals. In one such example, selectionmodule 915 applies predetermined or other weights to the first andsecond indicated pacing intervals to compute the selected pacinginterval S_(n).

Controller Example 2

[0076]FIG. 10 is a schematic diagram illustrating generally, by way ofexample, but not by way of limitation, another conceptualization ofportions of controller 325, with certain differences from FIG. 9 moreparticularly described below. In FIG. 10, controller 325 includes anatrial tachyarrhythmia (AT) detection module 1000 that receives a signalfrom atrial sensing circuit 305. The received signal includesinformation about atrial events, from which AT detection module 1000determines the presence or absence of one or more atrialtachyarrhythmias, such as atrial fibrillation.

[0077] In one embodiment, AT detection module 1000 provides a controlsignal, to pacing control module 505, that indicates the presence orabsence of an atrial tachyarrhythmia, such as atrial fibrillation. Inone embodiment, selection module 915 selects between the first andsecond indicated pacing intervals as illustrated, by way of example, butnot by way of limitation, in Table 1. TABLE 1 Example Selection Based onAT Detection, 1st Indicated Pacing Interval, Interval, and 2nd IndicatedPacing Interval AT Present? 1st Indicated Pacing 1st Indicated PacingInterval <2nd Indicated Interval ≧2nd Indicated Pacing Interval? PacingInterval? Yes, AT Present S_(n) ← 1st Indicated Pacing S_(n) ← 2ndIndicated Interval (i.e., VRR) Pacing Interval (e.g., Sensor) No, AT notS_(n) ← 2nd Indicated S_(n) ← 2nd Indicated Present Pacing Interval(e.g., Pacing Interval (e.g., Sensor) Sensor)

[0078] In this embodiment, if an atrial tachyarrhythmia is present andthe first indicated pacing interval is shorter than the second indicatedpacing interval, then selection module 915 selects the first indicatedpacing interval, which is based on the VRR techniques described above,as the selected indicated pacing interval S_(n). Otherwise, selectionmodule 915 selects the second indicated pacing interval, which in oneembodiment is based on the sensor indications, as the selected indicatedpacing interval S_(n). As discussed above, if no ventricular beat issensed during the selected indicated pacing interval S_(n), which ismeasured as the time from the occurrence of the ventricular beatconcluding the most recent V-V interval VV_(n), then pacing controlmodule 505 instructs ventricular therapy circuit 320 to deliver aventricular pacing pulse upon the expiration of the selected indicatedpacing interval S_(n).

[0079] Stated differently, for this embodiment, the ventricle is pacedat the VRR indicated rate only if an atrial tachyarrhythmia, such asatrial fibrillation, is present and the VRR indicated rate exceeds thesensor indicated rate. Otherwise the ventricle is paced at the sensorindicated rate. If, for example, the patient is resting, such that thesensor indicated rate is lower than the patient's intrinsic rate, and noatrial tachyarrhythmia is present, then the device will sense theintrinsic rate or will deliver ventricular paces at the lower ratelimit. But if, for example, the patient is resting, but with an atrialtachyarrhythmia that induces irregular ventricular contractions, thenpacing pulses generally will be delivered at the VRR indicated rate. Inanother example, if the patient is active, such that the sensorindicated rate is higher than the VRR indicated rate, then pacing pulsesgenerally will be delivered at the sensor indicated rate, whether or notatrial tachyarrhythmia is present. As an alternative to the selectiondescribed with respect to Table 1, selection module 915 provides a fixedor variable weighting or blending of both the sensor-indicated rate andVRR indicated rate, such that pacing pulses are delivered based on theblended rate.

[0080] The second indicated pacing interval need not be based on sensorindications. In one embodiment, for example, the second indicated pacinginterval tracks the sensed atrial heart rate when no atrialtachyarrhythmia is present. In this embodiment, selection module 915performs a mode-switching function in which the first indicated pacinginterval is used whenever atrial tachyarrhythmia is present and thesecond indicated pacing interval (e.g., atrial-tracking) is used when noatrial tachyarrhythmia is present.

[0081] In another embodiment, heart rate/interval is used as a triggerto turn on/off use of the first indicated pacing interval (e.g., the VRRindicated pacing interval). In one example, pacing therapy is based onthe first indicated pacing interval if the first indicated pacinginterval is longer than a first predetermined value, and pacing therapyis substantially independent of the first indicated pacing interval ifthe first indicated pacing interval is shorter than the firstpredetermined value. In this example, the VRR indicated pacing intervalis used at low heart rates, but not at fast heart rates.

Filter Rate Behavior Example 1

[0082]FIG. 11 is a graph illustrating generally, by way of example, butnot by way of limitation, one embodiment of a VRR indicated rate forsuccessive ventricular heart beats for one mode of operating filter 515.As discussed above, the VRR indicated rate is simply the frequency,between ventricular heart beats, associated with the first indicatedpacing interval. Stated differently, the VRR indicated rate is theinverse of the duration of the first indicated pacing interval. Ifpacing is based solely on the VRR indicated rate, pacing control module505 directs ventricular therapy circuit 320 to issue a pacing pulseafter the time since the last ventricular beat equals or exceeds thefirst indicated pacing interval. However, as described above, in certainembodiments, pacing control module 505 directs ventricular therapycircuit 320 to issue a pacing pulse based on factors other than the VRRindicated rate such as for, example, based on the sensor indicated rate.

[0083] In the example illustrated in FIG. 11, a first sensed intrinsicventricular beat, indicated by an “S” was detected just beforeexpiration of the first indicated pacing interval (“VRR indicated pacinginterval”) T₀, as computed based on a previous ventricular beat. In oneembodiment, the new VRR indicated pacing interval T₁ is computed basedon the duration of most recent V-V interval VV₁ and a previous value ofthe VRR indicated pacing interval T₀, as discussed above. In thisexample, the new VRR indicated pacing interval T₁ corresponds to a lowerrate limit (LRL) time interval. In one embodiment, the allowable rangeof the VRR indicated pacing interval is limited so that the VRRindicated pacing interval does not exceed the duration of the LRL timeinterval, and so that the VRR indicated pacing interval is not shorterthan the duration of an upper rate limit (URL) time interval.

[0084] The second ventricular beat is also sensed, just beforeexpiration of the VRR indicated pacing interval T₁. In one embodiment,the new VRR indicated pacing interval T₂ is computed based on theduration of most recent V-V interval VV₂ and a previous value of the VRRindicated pacing interval, T₁, as discussed above. The first and secondventricular beats represent a stable intrinsic rhythm, for which nopacing is delivered because the VRR indicated pacing interval is at alower rate than the sensed intrinsic ventricular beats.

[0085] The third, fourth, and fifth ventricular beats represent theonset of atrial fibrillation, resulting in erratic ventricular rates.The third ventricular beat is sensed well before expiration of the VRRindicated pacing interval T₂, such that no pacing pulse is issued. Forthe sensed third ventricular beat, filter 515 computes the new VRRindicated pacing interval T₃ as being shorter in duration relative tothe previous VRR indicated pacing interval T₂.

[0086] The fourth ventricular beat is similarly sensed well beforeexpiration of the VRR indicated pacing interval T₃, such that no pacingpulse is issued. For the sensed fourth ventricular beat, filter 515computes the new VRR indicated pacing interval T₄ as being shorter induration relative to the previous VRR indicated pacing interval T₃.

[0087] The fifth ventricular beat is sensed before expiration of the VRRindicated pacing interval T₄, such that no pacing pulse is issued. Forthe sensed fifth ventricular beat, filter 515 computes the new VRRindicated pacing interval T₅ as being shorter in duration relative tothe previous VRR indicated pacing interval T₄.

[0088] The sixth, seventh, and eighth ventricular beats indicateregularization of the ventricular rate using the pacing techniquesdescribed above. No ventricular beat is sensed during the VRR indicatedpacing interval T₅, so a pacing pulse is issued to evoke the sixthventricular beat. A new VRR indicated pacing interval T₆ is computed asbeing increased in duration relative to the previous VRR indicatedpacing interval T₅, lowering the VRR indicated rate. Similarly, noventricular beat is sensed during the VRR indicated pacing interval.

[0089] The ninth ventricular beat represents another erratic ventricularbeat resulting from the atrial fibrillation episode. The ninthventricular beat is sensed before expiration of the VRR indicated pacinginterval T₈. As a result, a shorter new VRR indicated pacing interval T₉is computed.

[0090] The tenth and eleventh ventricular beats illustrate furtherregularization of the ventricular rate using the pacing techniquesdescribed above. No ventricular beat is sensed during the VRR indicatedpacing interval T₉, so a pacing pulse is issued to evoke the tenthventricular beat. A new VRR indicated pacing interval T₁₀ is computed asbeing increased in duration relative to the previous VRR indicatedpacing interval T₉, lowering the VRR indicated rate. Similarly, noventricular beat is sensed during the VRR indicated pacing interval T₁₀,so a pacing pulse is issued to evoke the tenth ventricular beat. A newVRR indicated pacing interval T₁₁ is compute as being increased induration relative to the previous VRR indicated pacing interval T₁₀,lowering the VRR indicated rate.

[0091] The twelfth, thirteenth, fourteenth, and fifteenth ventricularbeats illustrate resumption of a stable intrinsic rhythm aftertermination of the atrial fibrillation episode. For such a stable rate,the VRR indicated rate proceeds asymptotically toward a “floor value”that tracks, but remains below, the intrinsic rate. This allows theintrinsic heart signals to control heart rate when such intrinsic heartsignals provide a stable rhythm. As a result, when the patient'sintrinsic rate is constant, paces will be withheld, allowing thepatient's intrinsic heart rhythm to continue. If the patient's heartrate includes some variability, and the VRR indicated floor value isclose to the mean intrinsic heart rate, then occasional paced beats willoccur. Such pace beats will gradually lengthen the VRR indicated pacinginterval, thereby allowing subsequent intrinsic behavior when thepatient's heart rate becomes substantially constant.

[0092] The intrinsic coefficient a of filter 515 controls the “attackslope” of the VRR indicated heart rate as the VRR indicated heart rateincreases because of sensed intrinsic beats. The paced coefficient b offilter 515 controls the “decay slope” of the VRR indicated heart rate asthe VRR indicated heart rate decreases during periods of paced beats. Inone embodiment, in which a>1.0 and b>1.0, decreasing the value of atoward 1.0 increases the attack slope such that the VRR indicated rateincreases faster in response to sensed intrinsic beats, while decreasingthe value of b toward 1.0 decreases the decay slope such that the VRRindicated rate decreases more slowly during periods of paced beats.Conversely, for a>1.0 and b>1.0, increasing the value of a from 1.0decreases the attack slope such that the VRR indicated rate increasesmore slowly in response to sensed intrinsic beats, while increasing thevalue of b from 1.0 increases the decay slope such that theVRR-indicated rate decreases more quickly during periods of paced beats.

[0093] In one embodiment, for a>1.0 and b>1.0, decreasing both a and btoward 1.0 increases VRR indicated rate during periods of sensedintrinsic activity so that the VRR indicated rate is closer to the meanintrinsic rate. Because the VRR indicated rate is closer to the meanintrinsic rate, variability in the intrinsic heart rate is more likelyto trigger paces at the VRR indicated rate. On the other hand, for a>1.0and b>1.0, increasing both a and b from 1.0 decreases the VRR indicatedrate during periods of sensed intrinsic activity so that the VRRindicated rate is farther beneath the mean intrinsic rate. Because theVRR indicated rate is farther beneath the mean intrinsic rate, the samevariability in the intrinsic heart rate becomes less likely to triggerpaces at the VRR indicated rate.

[0094] In one embodiment, these coefficients are programmable by theuser, such as by using remote programmer 125. In another embodiment, theuser selects a desired performance parameter (e.g., desired degree ofrate regularization, desired attack slope, desired decay slope, etc.)from a corresponding range of possible values, and device 105automatically selects the appropriate combination of coefficients offilter 515 to provide a filter setting that corresponds to the selecteduser-programmed performance parameter, as illustrated generally by Table2. Other levels of programmability or different combinations ofcoefficients may also be used. TABLE 2 Example of Automatic Selection ofAspects of Filter Setting Based on a User-Programmable PerformanceParameter. User-Programmable Performance Parameter Intrinsic Coefficienta Paced Coefficient b 1 (Less Rate 2.0 3.0 Regularization) 2 1.8 2.6 31.6 2.2 4 1.4 1.8 5 1.2 1.4 6 (More Rate 1.0 1.0 Regularization)

Filter Rate Behavior Example 2

[0095]FIG. 12 is a graph illustrating generally, by way of example, butnot by way of limitation, one embodiment of selecting between more thanone indicated pacing interval. FIG. 12 is similar to FIG. 11 in somerespects, but FIG. 12 includes a second indicated pacing interval. Inone embodiment, the first indicated pacing interval is the VRR indicatedpacing interval, described above, and the second indicated pacinginterval is a sensor indicated pacing interval, from an accelerometer,minute ventilation, or other indication of the patient's physiologicalneed for increased cardiac output.

[0096] In one embodiment, a selected indicated pacing interval is basedon the shorter of the first and second indicated pacing intervals.Stated differently, device 105 provides pacing pulses at the higherindicated pacing rate. In the example illustrated in FIG. 12, first andsecond beats and the twelfth through fifteenth beats are paced at thesensor indicated rate, because it is higher than the VRR indicated rateand the intrinsic rate. The third, fourth, fifth, and ninth beats aresensed intrinsic beats that are sensed during the shorter of either ofthe VRR and sensor indicated pacing intervals. The sixth through eighthbeats and tenth and eleventh beats are paced at the VRR indicated rate,because it is higher than the sensor indicated rate. Also, for thesebeats, no intrinsic beats are sensed during the VRR indicated intervals.In one embodiment, the above-described equations for filter 515 operateto increase the VRR indicated rate toward the sensor-indicated rate whenthe sensor indicated rate is greater than the VRR indicated rate, asillustrated by first through third and twelfth through fifteenth beatsin FIG. 12. In an alternate embodiment, however,T_(n)=b·w·VV_(n)+(1−w)·T_(n-1), if VV_(n) is concluded by a VRRindicated paced beat, and T_(n)=T_(n-1) if VV_(n) is concluded by asensor indicated paced beat, thereby leaving the VRR indicated rateunchanged for sensor indicated paced beats.

[0097] In this embodiment, the ranges of both the sensor indicated rateand the VRR indicated rate are limited so that they do not extend torates higher than the URL or to rates lower than the LRL. In oneembodiment, the LRL and the URL are programmable by the user, such as byusing remote programmer 125.

[0098] In a further embodiment, the selected indicated pacing intervalis based on the shorter of the first and second indicated pacingintervals only if an atrial tachyarrhythmia, such as atrialfibrillation, is present. Otherwise, the second indicated pacinginterval is used, as described above.

Filter Rate Behavior Example 3

[0099]FIG. 13 is a graph illustrating generally, by way of example, butnot by way of limitation, another illustrative example of heart rate vs.time according to a spreadsheet simulation of the behavior of theabove-described VRR algorithm. In FIG. 13, the VRR algorithm is turnedoff until time 130. Stable intrinsic lower rate behavior is modeled fortimes between 0 and 10 seconds. Erratic intrinsic ventricular rates,such as would result from atrial tachyarrhythmias including atrialfibrillation, are modeled during times between 10 seconds and 130seconds. At time 130 seconds, the VRR algorithm is turned on. While someerratic intrinsic beats are subsequently observed, the VRR algorithmprovides pacing that is expected to substantially stabilize the heartrate, as illustrated in FIG. 13. The VRR indicated pacing rate graduallydecreases until intrinsic beats are sensed, which results in a slightincrease in the VRR indicated pacing rate. Thus, the VRR algorithmfavors the patient's intrinsic heart rate when it is stable, and pacesat the VRR indicated heart rate when the patient's intrinsic heart rateis unstable. It is noted that FIG. 13 does not represent clinical data,but rather provides a simulation model that illustrates one example ofhow the VRR algorithm is expected to operate.

Filter Example 4

[0100] In one embodiment, filter 515 includes variable coefficients suchas, for example, coefficients that are a function of heart rate (or itscorresponding time interval). In one example, operation of the filter515 is described by T_(n)=a·w·VV_(n)+(1−w)·T_(n-1), if VV_(n) isconcluded by an intrinsic beat, otherwise is described byT_(n)=b·w·VV_(n)+(1−w)·T_(n-1), if VV_(n) is concluded by a paced beat,where at least one of a and b are linear, piecewise linear, or nonlinearfunctions of one or more previous V-V intervals such as, for example,the most recent V-V interval, VV_(n).

[0101]FIG. 14 is a graph illustrating generally, by way of example, butnot by way of limitation, one embodiment of using at least one ofcoefficients a and b as a function of one or more previous V-V intervalssuch as, for example, the most recent V-V interval, VV_(n). In one suchexample, a is less than 1.0 when VV_(n) is at or near the lower ratelimit (e.g., 1000 millisecond interval or 60 beats/minute), and a isgreater than 1.0 when VV_(n) is at or near the upper rate limit (e.g.,500 millisecond interval or 120 beats/minute). For a constant b, using asmaller value of a at lower rates will increase the pacing rate morequickly for sensed events; using a larger value of a at higher ratesincreases the pacing rate more slowly for sensed events. In anotherexample, b is close to 1.0 when VV_(n) is at or near the lower ratelimit, and b is greater than 1.0 when VV_(n) is at or near the upperrate limit. For a constant a, using a smaller value of b at lower rateswill decrease the pacing rate more slowly for paced events; using alarger value of b at higher rates decreases the pacing rate more quicklyfor paced events.

Biventricular Coordination Therapy Example

[0102] In one embodiment, the present cardiac rhythm management systemutilizes the VRR filter 515 for providing biventricular pacingcoordination therapy in both right and left ventricles, therebycoordinating the contractions of the right and left ventricles for moreefficient pumping. The VRR filter 515 controls the timing of delivery ofbiventricular pacing pulses. Filter 515 provides a first indicatedpacing rate that is independent of (i.e., does not track) the intrinsicatrial rate. The first indicated pacing rate is generally above the meanintrinsic ventricular rate so that it can provide substantiallycontinuous pacing therapy. This is advantageous because, among otherthings, either or both of the intrinsic atrial or intrinsic ventricularheart rates may be extremely irregular, such as during atrialtachyarrhythmias. The VRR techniques described above with respect toFIGS. 1-14 promote intrinsic ventricular beats when the ventricularheart rate is substantially constant. In contrast to such VentricularRate Regularization that promotes intrinsic activity, biventricularcoordination therapy typically uses Ventricular Rate Regulation thatprovides nearly continuous (i.e., to the desired degree) biventricularpacing when the ventricular rate is substantially constant. Oneselection of coefficients for filter 515 for obtaining nearly continuouspacing is described below.

[0103]FIG. 15 is a schematic drawing, similar to FIG. 2, illustratinggenerally by way of example, but not by way of limitation, oneembodiment of a cardiac rhythm management device 105 coupled by leads110A-C to a heart 115. In one such embodiment, system 100 providesbiventricular coordination therapy to coordinate right ventricular andleft ventricular contractions, such as for congestive heart failurepatients. FIG. 15 includes a left ventricular lead 110C, insertedthrough coronary sinus 220 and into the great cardiac vein so that itselectrodes, which include electrodes 1500 and 1505, are associated withleft ventricle 205B for sensing intrinsic heart signals and providingone or more of coordination paces or defibrillation shocks.

[0104]FIG. 16 is a schematic drawing, similar to FIG. 3, illustratinggenerally by way of example, but not by way of limitation, oneembodiment of portions of a cardiac rhythm management device 105, inwhich the left ventricular lead is coupled by lead 110C to a leftventricular sensing circuit 1600 and a left ventricular therapy circuit1605, each of which are, in turn, coupled by node/bus 327 to controller325. This embodiment also includes an atrial therapy circuit 1610, and aright ventricular lead 110B coupling right ventricle 205A to rightventricular sensing circuit 310 and right ventricular therapy circuit320, each of which are, in turn, coupled by node/bus 327 to controller325.

[0105]FIG. 17 is a schematic drawing, similar to FIG. 5, illustratinggenerally by way of example, but not by way of limitation, portions ofone conceptual embodiment of controller 325. In this embodiment,ventricular event module 500 receives input signals from rightventricular sensing circuit 310 and left ventricular sensing circuit1600. Pacing control module 505 provides control signals to rightventricular therapy circuit 320 and left ventricular therapy circuit1605. Operation of filter 515 is similar to the above descriptionaccompanying FIGS. 5-14, with certain differences discussed below,thereby allowing device 105 to provide biventricular coordinationtherapy at a VRR-indicated rate, a sensor-indicated rate, or acombination thereof.

[0106] In one embodiment, ventricular event module 500 detects sensedand paced ventricular beats from both right ventricular sensing circuit310 and left ventricular sensing circuit 1600. An interval betweensuccessive ventricular events, referred to as a V-V interval, isrecorded by a first timer, such as V-V interval timer 510. Ventricularevent module 500 selects the particular ventricular events initiatingand concluding the V-V interval timed by V-V interval timer 510. In afirst mode of operation, the V-V interval is initiated by a rightventricular beat (paced or sensed), and the V-V interval is thenconcluded by the next right ventricular beat (aced or sensed). In asecond mode of operation, the V-V interval is initiated by a leftventricular beat (paced or sensed), and the V-V interval is thenconcluded by the next left ventricular beat (paced or sensed). In athird mode of operation, the V-V interval is initiated by either a rightor left ventricular beat, and the V-V interval is then concluded by thenext right or left ventricular beat that occurs after expiration of arefractory period of approximately between 130 milliseconds and 500milliseconds (e.g., 150 milliseconds). Left or right ventricular beatsoccurring during the refractory period are ignored. Using the refractoryperiod ensures that the beat concluding the V-V interval is associatedwith a subsequent ventricular contraction, rather than a depolarizationassociated with the same ventricular contraction, in which thedepolarization is merely sensed in the opposite ventricle from theinitiating beat. Such a refractory period can also be used inconjunction with the first mode (V-V interval initiated and concluded byright ventricular beats) or the second mode (V-V interval initiated andconcluded by left ventricular beats).

[0107] Filter 515 computes a “first indicated pacing interval,” i.e.,one indication of a desired time interval between ventricular events or,stated differently, a desired ventricular heart rate. Based on the firstindicated pacing interval stored in register 520, pacing control module505 delivers control signals to one or more of therapy circuits 320 and340 for delivering therapy, such as biventricular pacing coordinationstimuli to one or more of right ventricle 205A and left ventricle 205B,at the VRR indicated ventricular heart rate corresponding to the inverseof the duration of the first indicated pacing interval.

[0108] Ventricular event module 500 also includes an output node/bus1700 coupled to pacing control module 505. Ventricular event module 500communicates to pacing control module 505 information about theoccurrence (e.g., timing, origin, etc.) of right and left ventricularsensed beats, so that pacing control module 505 can issue biventricularcoordination therapy to obtain coordinated right and left ventricularcontractions. In one embodiment, a sensed right ventricular contractiontriggers an immediate or very slightly delayed left ventricular pacingpulse, either alone, or in conjunction with a right ventricular pacingpulse. Similarly, a sensed left ventricular contraction triggers animmediate or very slightly delayed right ventricular pacing pulse,either alone or in conjunction with a left ventricular pacing pulse.This ensures that contractions of the right and left ventricles arecoordinated to provide more efficient pumping of blood by the heart 115.In one embodiment, the controller illustrated in FIG. 17 includes asensor channel, as discussed above with respect to FIG. 9. In thisembodiment, device 105 provides biventricular coordination therapy at aVRR-indicated rate, a sensor-indicated rate, or a combination thereof.

[0109] In one embodiment, the coefficients of filter 515 areprogrammable by the user, such as by using remote programmer 125, asdescribed above, in order to obtain a desired degree of pacing vs.sensing. In another embodiment, the user selects a desired performanceparameter (e.g., desired degree of pacing vs. sensing, etc.) from acorresponding range of possible values, and device 105 automaticallyselects the appropriate combination of coefficients of filter 515 toprovide a filter setting that corresponds to the selecteduser-programmed performance parameter, as illustrated generally by Table3. Other levels of programmability or different combinations ofcoefficients may also be used. TABLE 3 Example of Automatic Selection ofAspects of Filter Setting Based on a User-Programmable PerformanceParameter Such as For Providing Biventricular Coordination Therapy.User-Programmable Performance Parameter Intrinsic Coefficient a PacedCoefficient b 1 (More Pacing) 0.6  1.05 2 0.7 1.2 3 0.8 1.3 4 0.9 1.4 5(Less Pacing) 1.0 1.5

[0110] In a further embodiment, device 105 uses a mapping, such asillustrated in Table 3, in a feedback control loop to automaticallyselect the “performance parameter” of Table 3 and correspondingcoefficients. The user programs a mean pacing frequency goal. Device 105measures the mean pacing frequency over a predetermined period of timeor predetermined number of V-V intervals. The measured mean pacing iscompared to the mean pacing frequency goal. If the measured mean pacingfrequency is higher than the goal mean pacing frequency, the performanceparameter in Table 3 is incremented/decremented toward less pacing.Conversely, if the measured mean pacing frequency is lower than the goalmean pacing frequency, the performance parameter in Table 3 isincremented/decremented toward more pacing. In a further embodiment, themeasured mean pacing frequency is compared to values that are slightlyoffset about the goal mean pacing frequency (e.g., goal mean pacingfrequency ± Δ) to provide a band of acceptable measured mean pacingfrequencies within which the performance parameter is not switched.

AV delay Regulation Embodiment for Congestive Heart Failure Patients

[0111] The preceding embodiment illustrated techniques of providing rateregulation for delivering biventricular coordination therapy tocongestive heart failure patients. Such techniques are particularlyadvantageous in the presence of atrial tachyarrhythmias, such as atrialfibrillation. When atrial tachyarrhythmias are present, atrial ratetracking would result in too-fast and irregular biventricularcoordination therapy. Because atrial tachyarrhythmias often induceirregular ventricular cardiac cycle lengths, ventricular tracking wouldalso produce erratic results if the above-described VRR techniques arenot used.

[0112] Where no atrial tachyarrhythmias are present, however,biventricular coordination therapy based on atrial rate tracking ispossible. The above-disclosed techniques are still useful, however, forproviding a first indicated timing interval. In one example, the firstindicated timing interval is a filter indicated atrioventricular (AV)delay based on the intrinsic AV delay, i.e., the interval between asensed P wave and a successively sensed R wave. In this example,biventricular coordination therapy is provided based on the tracking ofan atrial rate. Each biventricular coordination pacing pulse isdelivered after the filter indicated AV delay. The filter indicated AVdelay is computed similarly to the VRR techniques described above, withcertain differences described more particularly below.

[0113]FIG. 18 is a schematic diagram, similar to FIG. 17, illustratinggenerally by way of example, but not by way of limitation, anotherconceptualization of portions of controller 325 used for regulating theAV interval based on a filter indicated AV delay. In FIG. 18, atrialevent module 1800 and ventricular event module 500 provide informationabout paced or sensed atrial events and paced or sensed ventricularevents, respectively, to AV interval timer 1805. AV interval timer 1805times an AV interval initiated by an atrial event, and concluded by aventricular event, such as described above with respect to FIG. 17.

[0114] In one example, operation of the filter 515 is described byT_(n)=a·w·AV_(n)+(1−w)·T_(n-1), if AV_(n) is concluded by an intrinsicbeat, otherwise is described by T_(n)=b·w·AV_(n)+(1−w)·T_(n-1), ifAV_(n) is concluded by a paced beat, where T_(n) is the newly computedvalue of the filter indicated AV interval, T_(n-1) is the previous valueof the filter indicated AV interval, AV_(n) is the time intervalcorresponding to the most recent AV delay period, and a, b, and w arecoefficients. In one embodiment, weighting coefficient w, intrinsiccoefficient a, and paced coefficient b, are variables. Differentselections of w, a, and b, will result in different operation of thepresent method and apparatus. For example, as w increases the weightingeffect of the most recent A-V interval AV_(n) increases and theweighting effect of the previous first indicated pacing rate T_(n-1)decreases. In one embodiment, w={fraction (1/16)}=0.0625. In anotherembodiment, w={fraction (1/32)}. Another possible range for w is fromw=½ to w={fraction (1/1024)}. A further possible range for w is from w≈0to w≈1. Other values of w, which need not include division by powers oftwo, may be substituted without departing from the present method andapparatus.

[0115] In one embodiment, intrinsic coefficient a, is selected to beless than (or, alternatively, less than or equal to) 1.0. In oneexample, the intrinsic coefficient a is selected to be lesser in valuethan the pacing coefficient b. In one embodiment, a≈0.6 and b≈1.5. Inanother embodiment, a=1.0 and b=1.05. One possible range for a is froma=0.6 to a=1.0, and for b is from b=1.05 to b=1.5. The coefficients mayvary without departing from the present method and apparatus.

[0116] In one embodiment, these coefficients are programmable by theuser, such as by using remote programmer 125. In another embodiment, theuser selects a desired performance parameter (e.g., desired degreepacing vs. sensing, desired attack slope, desired decay slope, etc.)from a corresponding range of possible values, and device 105automatically selects the appropriate combination of coefficients offilter 515 to provide a filter setting that corresponds to the selecteduser-programmed performance parameter, as illustrated generally by Table4. Other levels of programmability or different combinations ofcoefficients may also be used. TABLE 4 Example of Automatic Selection ofAspects of Filter Setting Based on a User-Programmable PerformanceParameter, Such as for AV Delay Regulation User-Programmable PerformanceParameter Intrinsic Coefficient a Paced Coefficient b 1 (Less Aggressive1.0  1.05 Attack/Decay) 2 0.9 1.2 3 0.8 1.3 4 0.7 1.4 5 (More Aggressive0.6 1.5 Attack/Decay)

[0117] In a further embodiment, device 105 uses a mapping, such asillustrated in Table 4, in a feedback control loop to automaticallyselect the “performance parameter” and corresponding coefficients. Theuser programs a mean sense frequency goal. Device 105 measures the meanfrequency of sensed ventricular events (“measured mean sense frequency”)over a predetermined period of time or predetermined number of A-Vintervals, and adjusts the performance parameter and correspondingcoefficients to direct the measured mean sense frequency toward the meansense frequency goal.

[0118] The above techniques provide an example in which AV delay isregulated. However, it is understood that these techniques extend to theregulation of any other timing interval. In one embodiment, for example,operation of filter 515 is expressed more generically asT_(n)=A·EE_(n)+B·t_(n-1), if EE_(n) is concluded by an intrinsic beat,otherwise is described by T_(n)=C·EE_(n)+D·T_(n-1), if EE_(n) isconcluded by a paced beat, where T_(n) is the newly computed value ofthe indicated timing interval, T_(n-1) is the previous value of theindicated timing interval, EE_(n) is the measured timing intervalbetween any two events, and A, B, C, and D are coefficients.

Conclusion

[0119] The above-described system provides, among other things, acardiac rhythm management system including techniques for computing anindicated pacing interval, AV delay, or other timing interval. In oneembodiment, a variable indicated pacing interval is computed based atleast in part on an underlying intrinsic heart rate. The indicatedpacing interval is used to time the delivery of biventricularcoordination therapy even when ventricular heart rates are irregular,such as in the presence of atrial fibrillation. In another embodiment, avariable filter indicated AV interval is computed based at least in parton an underlying intrinsic AV interval. The indicated AV interval isused to time the delivery of atrial tracking biventricular coordinationtherapy when atrial heart rhythms are not arrhythmic. Other indicatedtiming intervals may be similarly determined. The indicated pacinginterval, AV delay, or other timing interval can also be used incombination with a sensor indicated rate indicator.

[0120] It is to be understood that the above description is intended tobe illustrative, and not restrictive. Many other embodiments will beapparent to those of skill in the art upon reviewing the abovedescription. The scope of the invention should, therefore, be determinedwith reference to the appended claims, along with the full scope ofequivalents to which such claims are entitled.

What is claimed is:
 1. A method, including: obtaining actual timingintervals between cardiac events; computing a first indicated timinginterval based at least on a most recent actual timing interval durationand a previous value of the first indicated timing interval; andproviding pacing therapy, based on the first indicated timing interval.2. The method of claim 1 , in which computing the first indicated timinginterval includes differently weighting at least one of (1) the mostrecent actual timing interval duration, or (2) the previous value of thefirst indicated timing interval, if the most-recent actual timinginterval is concluded by a paced beat than if the most recent actualtiming interval is concluded by a sensed beat.
 3. The method of claim 1, in which computing the first indicated timing interval includessumming a first addend based on the most recent actual timing intervalduration and a second addend based on the previous value of the firstindicated timing interval, wherein at least one of the first and secondaddends is different if the most recent actual timing interval isconcluded by an intrinsic beat than if the most recent actual timinginterval is concluded by a paced beat.
 4. The method of claim 1 , inwhich computing the first indicated timing interval (T_(n)) is carriedout according to T_(n)=A·EE_(n)+B·T_(n-1), where A and B arecoefficients, EE_(n) is the most recent actual timing interval betweenevents, and T_(n-1) is the previous value of the first indicated timinginterval.
 5. The method of claim 4 , in which computing the firstindicated timing interval (T_(n)) is carried out according to:T_(n)=A·EE_(n)+B·T_(n-1) if EE_(n) is concluded by an intrinsic beat,otherwise is carried out according to T_(n)=C·EE_(n)+D·T_(n-1) if EE_(n)is concluded by a paced beat, where C and D are coefficients.
 6. Themethod of claim 5 , in which at least one of A, B, C, and D is afunction of heart rate.
 7. The method of claim 1 , in which computingthe first indicated timing interval (T_(n)) is carried out according toT_(n)=a·w·EE_(n)+(1−w)·T_(n-1) if EE_(n) is concluded by an intrinsicbeat, otherwise is carried out according toT_(n)=b·w·EE_(n)+(1−w)·T_(n-1), if EE_(n) is concluded by a paced beat,where a, b, and w are coefficients, EE_(n) is the most recent actualtiming interval duration, and T_(n-1) is the previous value of the firstindicated timing interval.
 8. The method of claim 7 , in which a isapproximately between 1.0 and 2.0, b is approximately between 1.0 and3.0, and w is approximately between 0 and
 1. 9. The method of claim 1 ,in which providing pacing therapy is also based on a second indicatedtiming interval that is based on a sensor.
 10. The method of claim 1 ,in which the first indicated timing interval is limited by at least oneof a maximum value and a minimum interval value.
 11. The method of claim1 , in which providing pacing therapy is based on the first indicatedtiming interval if atrial tachyarrhythmia is present, and providingpacing therapy is independent of the first indicated timing interval ifno atrial tachyarrhythmia is present.
 12. The method of claim 1 , inwhich computing the first indicated timing interval is also based on aremotely user-programmable parameter that corresponds to a desireddegree of paced beats vs. sensed beats.
 13. The method of claim 1 , inwhich computing the first indicated timing interval is also based on agoal proportion of paced beats to sensed beats.
 14. The method of claim1 , further including providing to a first ventricle a pace pulsetriggered by a sensed beat in a second ventricle different from thefirst ventricle.
 15. A method, including: obtaining atrio-ventricular(AV) intervals between atrial events and successive ventricular events;computing a first indicated AV interval based at least on a most recentAV interval duration and a previous value of the first indicated AVinterval; and providing pacing therapy, based on the first indicated AVinterval.
 16. The method of claim 15 , in which computing the firstindicated AV interval includes differently weighting at least one of (1)the most recent AV interval duration, or (2) the previous value of thefirst indicated AV interval, if the most-recent AV interval is concludedby a paced beat than if the most recent AV interval is concluded by asensed beat.
 17. The method of claim 15 , in which computing the firstindicated AV interval includes summing a first addend based on the mostrecent AV interval duration and a second addend based on the previousvalue of the first indicated AV interval, wherein at least one of thefirst and second addends is different if the most recent AV interval isconcluded by an intrinsic beat than if the most recent AV interval isconcluded by a paced beat.
 18. The method of claim 15 , in whichcomputing the first indicated AV interval (T_(n)) is carried outaccording to T_(n)=A·AV_(n)+B·T_(n-1) where A and B are coefficients,AV_(n) is the most recent AV interval, and T_(n-1) is the previous valueof the first indicated AV interval.
 19. The method of claim 18 , inwhich computing the first indicated AV interval (T_(n)) is carried outaccording to: T_(n)=A·AV_(n)+B·T_(n-1) if AV_(n) is concluded by anintrinsic beat, otherwise is carried out according toT_(n)=C·AV_(n)+D·T_(n-1), if AV_(n) is concluded by a paced beat, whereC and D are coefficients.
 20. The method of claim 19 , in which at leastone of A, B, C, and D is a function of heart rate.
 21. The method ofclaim 15 , in which computing the first indicated AV interval (T_(n)) iscarried out according to T_(n)=a·w·AV_(n)+(1−w)·T_(n-1) if AV_(n) isconcluded by an intrinsic beat, otherwise is carried out according toT_(n)=b·w·AV_(n)+(1−w)·T_(n), if AV_(n) is concluded by a paced beat,where a, b, and w are coefficients, AV_(n) is the most recent AVinterval duration, and T_(n-1) is the previous value of the firstindicated AV interval.
 22. The method of claim 15 , in which the firstindicated AV interval is limited by at least one of a maximum AVinterval value and a minimum AV interval value.
 23. The method of claim15 , in which providing pacing therapy is based on the first indicatedAV interval if no atrial tachyarrhythmia is present, and providingpacing therapy is independent of the first indicated AV interval ifatrial tachyarrhythmia is present.
 24. The method of claim 15 , in whichcomputing the first indicated AV interval is also based on a remotelyuser-programmable parameter that corresponds to a desired degree ofshortening vs. lengthening of AV interval.
 25. The method of claim 15 ,in which computing the first indicated AV interval is also based on agoal proportion of paced ventricular beats to sensed ventricular beats.26. A method, including: obtaining V-V intervals between ventricularbeats; computing a first indicated pacing interval based at least on amost recent V-V interval duration and a previous value of the firstindicated pacing interval; and providing pacing therapy to first andsecond ventricles, based on the first indicated pacing interval.
 27. Themethod of claim 26 , in which obtaining V-V intervals betweenventricular beats includes obtaining the V-V intervals betweenventricular beats of the same ventricle.
 28. The method of claim 26 , inwhich obtaining V-V intervals between ventricular beats includesobtaining the V-V intervals between ventricular beats of differentventricles.
 29. The method of claim 26 , in which obtaining V-Vintervals between ventricular beats includes: obtaining an initiatingventricular beat, in one of the first and second ventricles, theinitiating ventricular beat initiating a V-V interval; and obtaining aconcluding ventricular beat, in one of the first and second ventricles,the concluding ventricular beat being the next ventricular beat detectedafter expiration of a first time delay, and the concluding ventricularbeat concluding the V-V interval.
 30. The method of claim 26 , in whichcomputing the first indicated pacing interval includes differentlyweighting at least one of (1) the most recent V-V interval duration, or(2) the previous value of the first indicated pacing interval, if themost-recent V-V interval is concluded by a paced beat than if the mostrecent V-V interval is concluded by a sensed beat.
 31. The method ofclaim 26 , in which computing the first indicated pacing intervalincludes summing a first addend based on the most recent V-V intervalduration and a second addend based on the previous value of the firstindicated pacing interval, wherein at least one of the first and secondaddends is different if the most recent V-V interval is concluded by anintrinsic beat than if the most recent V-V interval is concluded by apaced beat.
 32. The method of claim 26 , in which computing the firstindicated pacing interval (T_(n)) is carried out according toT_(n)=A·VV_(n)+B·T_(n-1), where A and B are coefficients, VV_(n) is themost recent V-V interval duration, and T_(n-1) is the previous value ofthe first indicated pacing interval.
 33. The method of claim 32 , inwhich computing the first indicated pacing interval (T_(n)) is carriedout according to: T_(n)=A·VV_(n)+B·T_(n-1), if VV_(n) is concluded by anintrinsic beat, otherwise is carried out according toT_(n)=C·VV_(n)+D·T_(n-1), if VV_(n) is concluded by a paced beat, whereC and D are coefficients.
 34. The method of claim 33 , in which at leastone of A, B, C, and D is a function of heart rate.
 35. The method ofclaim 26 , in which computing the first indicated pacing interval(T_(n)) is carried out according to T_(n)=a·w·VV_(n)+(1−w)·T_(n-1) ifVV_(n) is concluded by an intrinsic beat, otherwise is carried outaccording to T_(n)=b·w·VV_(n)+(1−w)·T_(n-1), if VV_(n) is concluded by apaced beat, where a, b, and w are coefficients, VV_(n) is the mostrecent V-V interval duration, and T_(n-1) is the previous value of thefirst indicated pacing interval.
 36. The method of claim 26 , in whichproviding pacing therapy is also based on a second indicated pacinginterval that is based on a sensor.
 37. The method of claim 26 , inwhich providing pacing therapy is based on the first indicated pacinginterval, if an atrial tachyarrhythmia is present, and providing pacingtherapy is independent of the first indicated pacing interval if noatrial tachyarrhythmia is present.
 38. The method of claim 37 , in whichthe atrial tachyarrhythmia is atrial fibrillation.
 39. The method ofclaim 26 , in which providing pacing therapy is based on the firstindicated pacing interval if the first indicated pacing interval islonger than a first predetermined value, and providing pacing therapy isindependent of the first indicated pacing interval if the firstindicated pacing interval is shorter than the first predetermined value.40. The method of claim 26 , in which computing the first indicatedpacing interval is also based on a remotely user-programmable parameterthat corresponds to a desired degree of paced ventricular beats vs.sensed ventricular beats.
 41. The method of claim 26 , in whichcomputing the first indicated pacing interval is also based on a goalproportion of paced ventricular beats to sensed ventricular beats. 42.The method of claim 26 , in which the first indicated pacing interval islimited by at least one of a maximum interval value and a minimuminterval value.
 43. A cardiac rhythm management system, including: asensing circuit for sensing events; a controller, obtaining timingintervals between events and computing a first indicated timing intervalbased at least on a most recent actual timing interval duration and aprevious value of the first indicated timing interval; and a therapycircuit, providing pacing therapy based on the first indicated timinginterval.
 44. The system of claim 43 , including a filter that computesthe first indicated timing interval by differently weighting at leastone of (1) the most recent actual timing interval duration, or (2) theprevious value of the first indicated timing interval, if themost-recent actual timing interval is concluded by a paced beat than ifthe most recent actual timing interval is concluded by a sensed beat.45. The system of claim 44 , in which the filter includes coefficientsA, B, C, and D, and the filter computes the first indicated timinginterval (T_(n)) according to: T_(n)=A·EE_(n)+B·T_(n-1) if EE_(n) isconcluded by an intrinsic beat, otherwise is carried out according toT_(n)=C·EE_(n)+D·T_(n-1), if EE_(n) is concluded by a paced beat, whereEE_(n) is the most recent actual timing interval duration and T_(n-1) isthe previous value of the first indicated timing interval.
 46. Thesystem of claim 43 , further including a sensor, and in which thecontroller computes a second indicated timing interval based on signalsreceived from the sensor, and in which the therapy circuit providespacing therapy that is also based on the second indicated timinginterval.
 47. The system of claim 46 , further including a selectionmodule, selecting between the first and second indicated timingintervals to provide a selected timing interval.
 48. The system of claim43 , including a programmer remote from and communicatively coupled tothe controller, and in which the controller includes a first parameterthat is programmable by the programmer, and the programmer includes anindicator based on a second parameter received from the controller. 49.The system of claim 48 , in which the first indicated timing interval isalso based on the remotely user-programmable first parameter, whereinthe first parameter corresponds to a desired proportion of pacedventricular beats to sensed ventricular beats.
 50. The system of claim43 , in which the controller includes a goal proportion of pacedventricular beats to sensed ventricular beats, and the first indicatedtiming interval is also based on the goal.
 51. A cardiac rhythmmanagement system, including: an atrial sensing circuit; a ventricularsensing circuit; a controller, the controller including: anatrio-ventricular (AV) interval timer, obtaining AV intervals betweenatrial events and successive ventricular events; a first register, forstoring a first indicated AV interval; and a filter, updating the firstindicated pacing interval based on a most recent AV interval provided bythe AV interval timer and previous value of the first indicated AVinterval stored the first register; and a ventricular therapy circuit,providing pacing therapy based at least partially on the first indicatedAV interval.
 52. The system of claim 51 , in which the filterdifferently weighting at least one of (1) the most recent AV intervalduration, or (2) the previous value of the first indicated AV interval,if the most recent AV interval is concluded by a paced beat than if themost recent AV interval is concluded by a sensed beat.
 53. The system ofclaim 52 , in which the filter includes coefficients A, B, C, and D, andthe filter computes the first indicated AV interval (T_(n)) accordingto: T_(n)=A·AV_(n)+B·T_(n-1) if AV_(n) is concluded by an intrinsicbeat, otherwise is carried out according to T_(n)=C·AV_(n)+D·T_(n-1), ifAV_(n) is concluded by a paced beat, where AV_(n) is the most recent AVinterval duration and T_(n-1) is the previous value of the firstindicated AV interval.
 54. The system of claim 51 , in which the filterincludes a switch that disables the filter when the atrial sensingcircuit indicates that an atrial tachyarrhythmia is present.
 55. Thesystem of claim 51 , including a programmer remote from andcommunicatively coupled to the controller, and in which the controllerincludes a first parameter that is programmable by the programmer, andthe programmer includes an indicator based on a second parameterreceived from the controller.
 56. The system of claim 55 , in which thefirst indicated AV interval is also based on the remotelyuser-programmable first parameter, wherein the first parametercorresponds to a desired degree of shortening vs. lengthening of the AVinterval.
 57. The system of claim 51 , in which the controller includesa goal proportion of paced ventricular beats to sensed ventricularbeats, and the first indicated AV interval is also based on the goal.58. A cardiac rhythm management system, including: at least oneventricular sensing circuit; a controller, the controller including: anV-V interval timer, obtaining V-V intervals between successive events inat least one ventricle; a first register, for storing a first indicatedpacing interval; and a filter, updating the first indicated pacinginterval based on a most recent V-V interval provided by the VV intervaltimer and previous value of the first indicated pacing interval storedthe first register; and a ventricular therapy circuit, providing pacingtherapy to first and second ventricles based at least partially on thefirst indicated pacing interval.
 59. The system of claim 58 , in whichthe filter differently weighting at least one of (1) the most recent VVinterval duration, or (2) the previous value of the first indicatedpacing interval, if the most recent AV interval is concluded by a pacedbeat than if the most recent AV interval is concluded by a sensed beat.60. The system of claim 59 , in which the filter includes coefficientsA, B, C, and D, and the filter computes the first indicated pacinginterval (T_(n)) according to: T_(n)=A·VV_(n)+B·T_(n-1) if VV_(n) isconcluded by an intrinsic beat, otherwise is carried out according toT_(n)=C·VV_(n)+D·T_(n-1), if VV_(n) is concluded by a paced beat, whereVV_(n) is the most recent VV interval duration and T_(n-1) is theprevious value of the first indicated pacing interval.
 61. The system ofclaim 58 , further including a sensor, and in which the controllercomputes a second indicated pacing interval based on signals receivedfrom the sensor, and in which the therapy circuit provides pacingtherapy that is also based on the second indicated pacing interval. 62.The system of claim 58 , further including an atrial sensing circuit,and in which the filter includes a switch that enables the filter whenthe atrial sensing circuit indicates that an atrial tachyarrhythmia ispresent.
 63. The system of claim 58 , including a programmer remote fromand communicatively coupled to the controller, and in which thecontroller includes a first parameter that is programmable by theprogrammer, and the programmer includes an indicator based on a secondparameter received from the controller.
 64. The system of claim 63 , inwhich the first indicated pacing interval is also based on the remotelyuser-programmable first parameter, wherein the first parametercorresponds to a desired degree of paced ventricular beats vs. sensedventricular beats.
 65. The system of claim 58 , in which the controllerincludes a goal proportion of paced ventricular beats to sensedventricular beats, and the first indicated pacing interval is also basedon the goal.
 66. The system of claim 58 , further including a firstelectrode associated with the first ventricle and a second electrodeassociated with the second ventricle.
 67. The system of claim 58 , inwhich the first indicated pacing interval is limited by at least one ofa maximum interval value and a minimum interval value.